Radar system for detecting the surroundings with compensation of interfering signals

ABSTRACT

The invention relates to a frequency-modulated radar system for detecting the surroundings with compensation of interfering effects, the compensation being achieved by varying one of the following sizes:
         a) temporal distance between the transmitted frequency ramps and temporal gap, respectively, between the frequency ramps,   b) time from the start of the transmission ramp to the beginning of the scanning of the receiving signal,   c) frequency at the start of the transmitted frequency ramp, and   d) sign of the slope of the frequency ramps.

The invention relates to a radar system for detecting the surroundingswith means for compensating interfering signals. A system of this typefor monitoring the environment can be used e.g. in a motor vehicle, inwhich a driver assistance or safety function is provided.

Radar systems for detecting the surroundings of a motor vehicle areknown. With the detection of the surroundings a dynamic situationpicture of the traffic results which shows the distance and the relativespeed of the surrounding objects. The situation picture can present theinitial information of a driver assistance system, which takes over e.g.the longitudinal regulation of the vehicle or which serves forrecognizing danger situations. Interfering factors such as internal orexternal interfering irradiations, the radiation of radar systems ofother vehicles, trappings of the radar sensor system distort themeasurement data of surrounding objects and lead if necessary to amisdetection of objects. Thus, the function of a driver assistancesystem is severely disturbed.

It is the object of the present invention to indicate a radar system,which compensates interfering signals.

The radar system claimed here includes several independently combinablepossibilities for compensating interfering signals. For example.internal interfering frequencies, external interfering irradiations,trappings rank among the compensated interfering signals. For thecompensation the starting times of the transmitting and/or receivingintervals in relation to a regular interval are varied in time.

The radar system for detecting the surroundings claimed here is equippedwith transmitting means for the directed emission of transmission power,receiving means for the directed receipt of transmission power reflectedat objects and signal processing means for processing the receivedpower. The frequency of the radiated transmission power is modulatedsuch that the transmission power includes a sequence of linear frequencyramps with a slope which is identical at least with regard to theamount. Between the frequency ramps temporal gaps or sections withanother arbitrary frequency modulation can occur. In the signalprocessing means a mixture between a signal with the currenttransmitting frequency and the transmission power received by thereceiving means and reflected at objects takes place. The output signalof the mixture is scanned, if necessary after suitable preprocessing, Ntimes during at least one frequency ramp, N being the number of thesamples.

A two-dimensional discrete time-frequency-transformation is fully oronly partially determined via the respective N samples of K ramps. Here,K indicates the number of the scanned frequency ramps. The radar systemis designed in such a way that during transformation relative speeds andradial distances are associated to the two-dimensional frequency range,i.e. conclusions are drawn onto the relative speed and the radialdistance of the associated object from the two-dimensional frequency ofdetected signal power. For suppressing interfering effects at least oneof the following sizes is varied: the temporal distance of the frequencyramps and the temporal gap, respectively, between the frequency ramps,the time from the ramp start up to the beginning of the scanning of theN values scanned during a frequency ramp, the frequency at the rampstart, the sign of the slope of the frequency ramps.

In an advantageous embodiment of the invention the variation of thefrequency ramp characteristics is random or pseudo-random or determined.Thus, also interferences are compensated, which are caused by otherradar systems (e.g. at an oncoming vehicle), which work after the sameor another method.

In a preferred embodiment of the invention the two-dimensional discretetime-frequency-transformation is a two-dimensional discrete fouriertransformation. A special embodiment of the radar system provides thatwhen varying one of the mentioned sizes a non-linear filtration ofdiscrete signals is performed. By the variation only individual powervalues of the signals are disturbed. These are reduced or suppressed intheir effect by a filtration with a non-linear filter.

In a preferred embodiment of the invention for the non-linear filtrationan average value of the amount or of the power of predetermined signalsis formed. Signal values, whose amount or power exceed this averagevalue by a predetermined value, are corrected. The values are set on afixed value e.g. zero.

In a preferred embodiment of the invention the non-linear filtration isapplied in each case on the N samples of a frequency ramp.

Another embodiment provides that for the signal evaluation atwo-dimensional time-frequency-transformation is performed in two steps.In the first transformation step a one-dimensionaltime-frequency-transformation is calculated each via the N samples of afrequency ramp. This step is repeated for K-1 frequency ramps. Thenon-linear filtration is applied for K values each, which show the samefrequency value after the first discrete time-frequency-transformation.In a second transformation step a one-dimensional discretetime-frequency-transformation is calculated in each case via the Koutput values of the non-linear filtration.

In a special embodiment of the invention at least one of the followingsizes is varied via the ramps: time of the N values scanned during afrequency ramp relative to the ramp start or frequency at the ramp startor the sign of the slope of the frequency ramps. A two-dimensionaltime-frequency-transformation is performed in two steps, wherein in thefirst step a one-dimensional time-frequency-transformation is calculatedin each case via the N samples of a frequency ramp. For compensating thevarying ramp characteristics the result of the first discretetime-frequency-transformation is multiplied with a factor ê(j*a*b), thesize a considering the respective frequency with the first sample of afrequency ramp and the sign of its slope and b depending on thefrequency raster value.

Another embodiment of the radar system provides that the temporaldistance of the frequency ramps is varied in such a way that the delayof the ramp starting times to a temporally fixed raster representsapproximately a discrete equal distribution.

In particular the delay of the ramp starting times to a temporally fixedraster is varied in only such strong manner that the difference from afixed raster point to the ramp start and to the associated actual rampstart is smaller than a predetermined threshold value. The thresholdvalue is selected e.g. such that with the two-dimensionaltime-frequency-transformation the interfering effect is negligibly smalldue to the non-equidistant scanning.

A preferred embodiment of the radar system provides that the time of theN values sampled during a frequency ramp is varied relative to the rampstart in a discrete raster, wherein these raster distances are at leastpartially unequal to the scanning distances of the N samples. Falsesignals caused by trappings are thus formed in incoherent manner. Inparticular the raster distance is constant and half the size as theconstant scanning distance between the N samples.

In the following the invention will become apparent on the basis ofdrawings and examples of embodiments, in which

FIG. 1 shows block diagram of a radar system with a compensation ofinterfering signals.

FIG. 2 a) shows a frequency time diagram of transmitting and receivingsignal b) shows a temporal course of transmitting and receiving signal

FIG. 3 shows a block diagram of the signal processing withtwo-dimensional FFT

FIG. 4 shows a signal power outlined above the frequency, recorded withvariation in time of the ramp starting times Δt_1 and of the samplingtimes Δt_2, respectively.

In FIG. 1 the block diagram of a radar system is shown. The dashed linesindicate the trigger paths. Analogue paths are marked with bold lines.At the time t_0 a starting pulse for the measurement is emitted. Forrecording a data set the starting pulse is repeated K-times at the sametemporal distance. The temporal distance of the starting pulsescorresponds to the pulse repeating interval with a regular pulse-dopplerradar.

The delay unit dt_1 delays the starting signal subject to k. k is acontrol variable whose value is increased with each further startingsignal by 1 each up to a predetermined value K (k=0,1,2,3,. . . K). Withthe output signal of the delay unit Δt₁ 1(k) the starting point of thetransmission interval becomes t_S is set for a measurement k. A unit forgenerating linear frequency-modulated signals, the frequency rampgenerator FRG, is controlled by the output signal. The signal T_xemitted by the unit is divided. One part is amplified if necessary andis emitted as a transmitting signal via an antenna, the second part issupplied to a mixer M. The receiving signal R_x reflected at surroundingobjects is received if necessary via the same antenna and is equallysupplied to the mixer M. The output signal of the mixer M has thedifference frequency of the transmitting signal T_x and the receivingsignal R_x. The frequency of the output signal is proportional to thedistance of the detected object. The output signal of the mixer isfiltered in a band pass filter. The filtered signal is digitized with ananalog-digital converter ADC and is scanned for this purpose N times perfrequency ramp within a receiving interval t_E. The output signal Signal1 consists of N samples per ramp with K repetitions (number of thescanned ramps). The start of the receiving interval t_E related to thestarting point of the transmission interval t_S is determined by thedelay unit Δt_2(k) for a measurement k.

In FIG. 2 a the frequency for a transmission ramp Tx and an associatedreceiving signal Rx is outlined above the time. The temporal offset Δtof the receiving signal Rx, caused by the flight time of the radarradiation to the object and backwards, results in a frequency shift Δfof the receiving signal Rx towards the transmitting signal Tx. Thefrequency shift Δf is proportional to the distance of the object.

In FIG. 2 b the frequency of transmitting signal Tx and receiving signalRx is outlined above the time. The mixed receiving signal is scannedonly in a limited receiving interval. The signal energy with apredetermined frequency—the distance of an object is proportional toDf—is contained in the entire mixing region Tx-Rx, therefore, theposition of the receiving interval t_E can be varied in this range. Inaddition, the delay times Δt_1(k) and Δt_2(k) are shown in FIG. 2 b. Bythe delay Δt_1(k) the transmitting and receiving intervals are shifted(“jitted”) in equal measure. Here, the signal level of fixed interferingfrequencies (e.g. internal interfering frequencies) is reduced, as theyare detected with another phase position in each of the K rampintervals. The range of values of the delay Δt_1(k) is selected to besuch small that the level of the wanted signal is reduced only slightly.By the additional delay Δt_2(k) the transmitting and receiving intervalsare shifted relative to each other. Thereby, the signal level ofexternal interference sources as well as the signal level from trappingreceipt are reduced.

FIG. 3 shows the digital signal processing of the samples up to thedoppler spectra. Signal 1, consisting of N values, recorded with Krepetitions, is fed into the signal processing unit. A two-dimensionalFFT is realized by successive FFTs of lines and gaps of a matrix. Atemporary storage Ma for the matrix values is arranged between the firstand the second FFT. Before the first and the second FFT windowing of thedata signal takes place. This is shown in FIG. 3 by the blocks WIN. Tominimize the influence of external interference sources as far aspossible non-linear filters are provided before the first and the secondFFT.

During the filtration those samples, whose amount are higher than athreshold value, are replaced by a fixed value, e.g. zero. The thresholdvalue depends on the amount and power values, respectively, of thesamples.

The delay Δt_2(k) has an influence not to be neglected onto the phase ofthe signal after the first FFT. The frequency-dependent phase rotationis compensated by the compensator Δt_comp subject to the value Δt_2(k).At the end of the signal processing the doppler signal Signal 2 isreceived, which is composed of K values with N repetitions.

In FIG. 4 the simulation of the signal level after the first FFT isoutlined above the distance gates n, with n=1 . . . 39. The distancegates result from the digital scanning. At the distance gate 10 a target1 is and at the distance gate 30 an interference line 2 with fixedfrequency and identical signal level is shown. The simulation wasperformed for different delays for transmitting and receiving interval.The solid line shows the signal level, if no variation of thetransmitting and receiving intervals Δt_1=O and Δt_2=0 is provided.Approximately the same signal level is achieved for both power peaks. Ifthe transmitting and receiving intervals are shifted in equal measure bya variation of Δt_1 (k), the signal level of the interference line fallsby approx. 18 dB, as is shown in FIG. 4 with the fine dashed line.

If in addition the transmitting and receiving intervals are shiftedrelative to each other by a variation of Δt_2(k) the signal level of theinterference line equally falls in the example to a value which isapprox. 18 dB below the output level (FIG. 4, rough dashed line).

The signal level of the target object 1 at the distance gate 10 ismaintained with the variation of Δt_1 and Δt_2.

1. A radar system for detecting the surroundings with transmitting meansfor the directed emission of transmission power, receiving means for thedirected reception of transmission power reflected at objects and signalprocessing means for the processing the received power, wherein a) thefrequency of the emitted transmission power is modulated in such a waythat the transmission power includes a sequence of linear frequencyramps with a slope which is identical at least with regard to theamount, b) between the frequency ramps temporal gaps or sections withother frequency modulation can arise, c) in the signal processing meansa mixture between a signal with the current transmission frequency andthe transmission power received from the receiving means and reflectedat objects takes place, d) in the signal processing means the outputsignal of the mixture, if necessary after suitable preprocessing, isscanned N times during at least one frequency ramp, e) in the signalprocessing means a two-dimensional discretetime-frequency-transformation is determined fully or only partially eachvia the N samples of K ramps, K indicating the number of the scannedfrequency ramps, f) and the design is such that during thetransformation relative speeds and radial distances are associated tothe two-dimensional frequency range, i.e. from the two-dimensionalfrequency of detected signal power conclusions are drawn onto therelative speed and the radial distance of the associated object,characterized in that for suppressing the interference effects at leastone of the following sizes is varied via the ramps: temporal distance ofthe frequency ramps and the temporal gap, respectively, between thefrequency ramps, time from the ramp start to the beginning of thescanning of the N values scanned during a frequency ramp, frequency atthe ramp start, sign of the slope of the frequency ramps.
 2. A radarsystem according to claim 1, characterized in that the variation of thefrequency ramp characteristics is random or pseudo-random or isdetermined. 3-12. (canceled)
 13. A radar system according to claim 1,characterized in that when varying at least one of the frequency rampcharacteristics a non-linear filtration of discrete signals isperformed.
 14. A radar system according to claim 13, characterized inthat with the non-linear filtration an average value for the amount orthe power is formed from a plurality of predetermined signal values, andin that all values, whose amount or power exceed this average value by apredetermined value, are corrected.
 15. A radar system according toclaim 13, in which the non-linear filtration is applied to the N samplesof a frequency ramp case.
 16. A radar system according to claim 13, inwhich the two-dimensional time-frequency-transformation is performed intwo steps, in the first transformation step a one-dimensional discretetime-frequency-transformation is calculated each via the N samples of afrequency ramp, this step being performed for K frequency ramps thenon-linear filtration is applied to K values, which have the samefrequency value after the first discrete time-frequency-transformationand in the second transformation step a one-dimensionaltime-frequency-transformation is calculated each via the K output valuesof the non-linear filtration.
 17. A radar system according to claim 1,characterized in that at least one of the following sizes is varied viathe ramps: a) time of the N values sampled during a frequency ramprelative to the ramp start b) frequency at the ramp start, c) sign ofthe slope of the frequency ramps, the two-dimensionaltime-frequency-transformation is performed in two steps, in the firststep a one-dimensional time-frequency-transformation being calculatedeach via N samples of a frequency ramp, and for compensating the varyingramp characteristics the result of the first discretetime-frequency-transformation is multiplied with a factor ê(j*a*b), thesize a depending on the respective frequency with the first sample of afrequency ramp and the sign of its slope being considered and bdepending on the frequency raster value.
 18. A radar system according toclaim 1, in which the temporal distance of the frequency ramps is variedvia the ramps in such a way that the delay of the ramp starting times toa temporally fixed raster represents approximately a discrete equaldistribution.
 19. A radar system according to claim 1, in which thedelay of the ramp starting times to a temporally fixed raster variesonly in such strong manner that the difference from a fixed raster pointto the ramp start and the associated actual ramp start is smaller than apredetermined threshold value.
 20. A radar system according to claim 1,in which the time of the N values sampled during a frequency ramp isvaried relative to the ramp start in a discrete raster, the rasterdistances being at least partially unequal to the scanning distances ofthe N samples.
 21. A radar system according to claim 20, in which thetime of the N values sampled during a frequency ramp is varied relativeto the ramp start in a discrete raster, the raster distance beingconstant and half the size as the constant scanning distance between theN samples.
 22. A driver assistance system, which includes a radar systemaccording to claim 1.